Chromium plating from a fluosilicate type bath containing sodium,ammonium and/or magnesium ions



United States Patent Oflice 3,514,380 Patented May 26, 1970 3 514,380 CHROMIUM PLATIN G FROM A FLUOSILICATE TYPE BATH CONTAINING SODIUM, AMlVIO- NIUM AND/ OR MAGNESIUM IONS Henry R. Tuchewicz, Parma Heights, and Thomas P. Malak, Garfield Heights, Ohio, assignors to Kewanee Oil Company, Bryn Mawr, Pa., a corporation of Delaware No Drawing. Filed Feb. 17, 1967, Ser. No. 616,762 Int. Cl. C23b 5/06 U.S. Cl. 20451 4 Claims ABSTRACT OF THE DISCLOSURE This invention comprises the use of certain electroplating bath additives in specific concentrations to improve the covering power when plating chromium from fluosilicate baths. Such fluosilicate baths contain chromium trioxide (200-380 grams per liter), sulfate ions (weight ratio of CrO /SO of between 150 to one to 230 to one) and fluosilicate ion (0.6-5.1 grams per liter). The additives which result in improved covering power and the effective concentrations of each are as follows:

Grams per liter M 2-22 Na+ 2-24 NH4+ 2-20 If mixtures of these additives are used, a total of between about 2.0 and 24 grams per liter would be needed. When plating microcracked chromium further addition of selenate ions to such baths increases the extent of microcracked chromium into areas of decreased current density.

instant invention were not present in the electroplating bath.

Originally commercial electroplating baths for the electrodeposition of chromium contained basically only chromic acid and sulfuric acid in a typical ratio of 100 to one. Later, it was discovered that the addition of fluosilicic acid to such a bath resulted in an improvement in efiiciency and in certain properties of the electrodeposited chromium. However, such fluosilicate type chromium plating baths in general tend to exhibit reduced covering power due to the fluosilicate present, i.e., the chromium from such a bath is not as readily plated onto areas of lower current density as it would be from a similar, bath containing no fluosilicate ions.

The present invention, however, effectively removes the covering power deficit of fluosilicate type chromium plating baths. The purpose of the present invention is realized by the inclusion of certain cations in limited amounts in such fluosilicate type baths. Thus, for example, in a fluosilicate type lbath containing chromic anhydride (CRO along with sulfate and fluosilicate (SiF ions in suitable concentrations, addition of sodium ion or other equally suitable cations to a suitable concentration has been found to appreciably improve the covering power of the bath with the result that chromium can be electrodeposited over a metal substrate onto areas having a lower current density than occurs when sodium ions are not present in the bath.

Other cations such as ammonium when used in suitable concentrations, produce similar improvement in covering power to that obtained when sodium ion is utilized. Magnesium ions also result in improved covering power and are included as part of the instant invention, but the extent of improvement obtainable using magnesium ions is less than when sodium, or ammonium ions are present in the fluosilicate type chromium electroplating bath.

Potassium ions, although similar chemically in other respects to the cations of the instant invention, are not included within the scope of the instant invention since it would be most difficult if not impossible to maintain and control the potassium and fluosilicate ion concentrations at accepta ble concentrations. Potassium has been utilized in the so-called self regulating fluosilicate type chromium electroplating baths but the relatively low, temperature dependent, solubility of potassium fluosilicate presents unsurmountable difficulties.

In greater detail, the present invention is applicable to fluosilicate type chromium plating wherein the bath composition is maintained within approximately the following limits as to concentration:

CrO 200-380 grams per liter.

SiF 0.65.1 grams per liter.

S0 Sufficient amount to result in a weight ratio of Gro /S0 of between to 1 and 230 to 1.

Mg++ 2-22 Na+ 2-24 NH 2-20 Anyone of the above cations when present in the above concentrations will increase the covering power of the bath and the cations listed can also be used in combination with equally as effective results. When a combination of more than one said cation is utilized in accordance with the present invention, the total cation concentration present will vary from about 2.0 to about 24 grams per liter depending on the particular cations selected.

When preparing a fluosilicate type electroplating bath, such as is useful in the present invention, chromic acid anhydride (CRO sulfuric acid (H 80 and fluosilicic acid (H SiF are the main sources of the desired components throughout the plating industry and may be used in the practice of the instant invention. Other sources of hexavalent chromium, sulfate ions or fluosilicate ions can be used and are readily obvious to those skilled in the art. The desirable cations to be added to such baths so as to enable one to practice the instant invention may be in any convenient form such as, for example, the chromate or hydroxide. If the sulfate of the desired cation is used, care should be taken to see that sulfate ion introduced in this manner to the solution is considered in determining the total sulfate ion concentration. Should excess sulfate ion be added to such a bath, the excess sulfate ion can be precipitated in the usual manner as barium sulfate by adding the required amount of barium carbonate or hydroxide.

When chromium electrodeposits of the order of 3 to about 30 millionths of an inch in thickness are desired for providing decorative and corrosion resisting characteristics over a metal substrate, it has been found that improved covering power of chromium due to use of the cation additives of the instant invention can be secured using electroplating baths containing between 200 and 380 grams per liter CrO 0.6 to 5.1 grams per liter fluosilicate ion and sufiicient sulfate ion to yield a CrO /SO weight ratio on the order of 150 to 230 to l. Preferably, when electroplating such decorative chromium deposits, the bath is maintained at 200 to 300 grams per liter CrO 0.6 to 3.8 grams per liter fiuosilicate ion and at a CrO /SO ratio of about 150 to 230 to 1. This is especially true when the decorative chromium is being electroplated over bright nickel. The improvement due to the cation additives of the instant invention is observed throughout the concentration ranges given earlier for such cations but is, in fact, dependent to some extent on the other components of the bath. For example, as the fluosilicate ion concentration increases in the range covered the cation additive concentration should also be increased within the range specified in this specification as to the particular cation utilized. For example, when sodium is the cation added to the bath, fluosilicate ion concentrations in the higher range requires 16 to 20 grams per liter Na+ to give optimum results, while at lower SiF concentrations only about 3 grams per liter Na+ is needed to maximize the covering power of the chromium being plated. This is evidenced by Examples II and IV of this specification which follow.

Tests have indicated that as the CrO content of the bath is increased beyond the upper concentration limit of the instant invention, 380 grams per liter, the additional CrO improves the covering power of the bath and the use of a cation additive such as sodium or the other cation additives of the instant invention may even tend to reduce the covering power. However, even though chromium electroplating has been performed commercially using baths containing CrO in excess of 380 grams per liter, such high concentrations are undesirable due to excessive costs in the use of chemicals, primarily due to dragout and similar losses.

In addition to its use in the relatively thin decorative, corrosion resistant chromium electroplate discussed above,

the instant invention is likewise applicable to thicker elec- 4 trocoatings of chromium as, for example, when producing a to millionths of an inch coating of microcracked chromium. In this type of deposit, a fine network of interlocking cracks is secured with a crack density of typically 200 to 2000 or more cracks per lineal inch. This type of chromium has been found to provide greatly improved corrosion resistance over that secured with the thinner conventional chromium deposit. Such microcracked chromium deposits can be obtained utilizing baths having concentrations of ingredients throughout the broad ranges of the instant invention although the higher SiF concentrations are preferred. Such preferred limits for microcracked chromium baths are 200 to 380 grams per liter CrO 2.5 to 5.1 grams per liter SiF and a cro /so.= ratio of 160 to 230 to one. The cation additive of the instant invention is added in the previously described concentrations2 to 24 grams per liter Na' 2 to 22 grams per liter Mg++, or 2 to 20 grams per liter NH This cation addition improves covering power while having no adverse effects on microcracking result and, in some instances, improved uniformity of the microcrack pattern is obtained.

A further improvement in the plating of microcracked chromium can be achieved by the further addition of a low concentration of a selenium compound. Such selenium compounds can be any which yield selenate ions (560 or are oxidized to the SeO in the plating bath. Such compounds include 8e0 SeO Na SeO Na SeO H SeO and H SeO and all appear to give comparable results at equivalent selenium concentrations. Other salts such as those of potassium, lithium and the like behave similarly. These selenium containing additives enable a given electroplating bath to provide microcracking to a greater extent in regions of decreased current density than is secured from a similar bath without the selenium. This improvement is noted at extremely low SeO concentrations, e.g. tests have indicated that as little as about 0.001 gram per liter Na ScO exhibited some extension of the microcracking into the lower current density areas. Further improvement in the extent of microcracking is achieved as the concentration of Na SeO is increased to about 0.006 gram per liter. Additional Na SeO up to a concentration of about 0.015 gram per liter does not change the extent of the crack pattern as the concentration is increased, and the microcrack pattern did become finer. Above a concentration of 0.015 gram per liter Na SeO the chromium deposit becomes hazy-blue and undesirable in appearance. Therefore, when adding selenium in the form of Na SeO the operable concentration range extends from less than 0.001 gram per liter to about 0.015 gram per liter, and the preferred range is between 0.004 and 0.010 gram per liter. When selenium is added in other forms, the above ranges are altered so as an equivalent selenium range is covered.

The following examples illustrate the present invention and clearly show the increased covering power and extension of microcracking by practice of the instant invention. In these examples, two types of tests are described. The first test method utilizes a bent cathode. The bent cathode is a standard V recess panel having the usual V shaped recess for determining the extent of misplate (i.e. no chromium deposit) measured from the bottom of the recess. Such bent cathode tests are illustrated further in Examples I to III regarding covering power of chromium plating and in Example VII regarding the extension of microcracking by practice of the instant invention.

The second test method utilizes a standard Hull Cell and in particular a 247 ml. Hull Cell. For comparing covering power of various chromium baths, brass panels upon which has been electroplated of the order of 0.0005 inch thickness of a semi-bright nickel are used as the cathode and chromium is deposited thereon for three minutes at a current of 5 amperes. The covering power of the chromium deposit is evaluated by measuring from the high density edge of the panel the distance over which the chromium has covered the panel.

EXAMPLE I A series of chromium plating baths were prepared in the usual manner from chromic acid anhydride, sulfuric acid and fiuosilicic acid to prepare baths having an initial composition of 253 grams per liter CrO 1.65 grams per liter SO and 3.7 grams per liter SiF To these baths were added varying amounts of sodium ion in the form of its hydroxide. The cathode used in each bath was 1 /2 inchs by 6 inches V recess brass panel, which had pre viously been electrodeposited with a 0.0005 inch thick bright nickel deposit. Each of these baths were maintained at about F. and the chromium was deposited on the cathode for eight minutes at a current of 10 amperes. The distances of misplate measured from the bottom of the recess for each bath were as follows:

Na+ added (g./l.): Misplate (mm) EXAMPLE II The procedure of Example I was repeated in baths having a composition of 253 grams per liter Cr 1.65 grams per liter SO.{ and 3.7 grams per liter SiF The V recess panels used were similar to those of Example I but measured 4 by 6 inches. Chromium was plated from these baths for 8 minutes at a current of 65 amperes. The results were as follows:

EXAMPLE III The procedure of Example I was repeated in baths having the initial bath composition of 227 grams per liter CrO" 1.2 grams per liter SO and 3.3 grams per liter SiF The results upon varying the sodium ion concentration were as follows: Na added (g./l.): Misplate (mm.) 0 15 3 11 7 7 9 7 EXAMPLE IV A number of 247 ml. Hull Cells were filled with a plating 'bath containing 227 grams per liter CrO 1.2 grams per liter S and 3.3 grams per liter SiF The cathodes used in these cells were brass panels upon which had been electroplated about a 0.0005 inch thickness of semi-bright nickel. The chromium was then deposited thereon for three minutes at a current of five amperes in the baths having varying concentrations of cation added thereto at bath temperatures of about 115 F. The results are reported as the distance down the plate from the high current density edge of the plate which was plated with chromium. The results were as follows:

Na+ added (g./l.): Distance covered (mm.) 0 77 3 79 EXAMPLE V The same results were obtained as in Example IV when the bath composition of Example IV was altered to 210 grams per liter CrO 1.05 grams per liter SO and 0.9 gram per liter Si-F All other parameters remained the same.

EXAMPLE VI Concen- Distance tration Covered Cation (g./l.) (mm.)

EXAMPLE VII In order to illustrate the effect of selenium in such cation containing fiuosilicate type chromium plating baths as defined by the instant invention, a series of baths were prepared containing 255 grams per liter CrO 1.275 grams per liter SO 3.0 grams per liter SiF and 3.5 grams per liter sodium ion. To these baths various amounts of seleniurn were added. The cathodes in these runs consisted of 4 by 6 inch V recess panels made of steel and having a 0.0005 inch electrodeposition of bright nickel thereon. Chromium was plated on each cathode for 8 minutes at a current of 65 amperes while the bath temperature was maintained at about F. The results were as follows:

Na SeO added Microcrack depth (mm.

In carrying out the plating process of the instant invention, the bath composition has been described in detail and is most important. However, the time, current and bath temperature should also be taken into consideration in any chromium plating operation. The present invention can lie utilized in plating chromium with increase covering power and/ or increased microcracking when the plating bath previously described in detail is maintained in the 100 to F. range. The plating operation can be completed in from one to fifteen minutes using a current density of 100 to 350 amperes per square foot.

The improved covering power due to the use of the cations of the instant invention in the chromium plating bath when used over nickel deposits will vary somewhat depending on the type and concentration of any brighteners 'which were present in the nickel plating bath. This is apparently due to the activity variance of the bright nickel deposit which is familiar to those skilled in the art.

While the present invention has been particularly described with respect to chromium electrodeposition over nickel deposits, such is not to be construed as limiting the instant invention. Other metal substrates, such as copper, steel, chromium and the like which are considered by those skilled in the art to be platable with chromium can be used as the metal substrates in the practice of the instant invention.

We claim:

1. A method of electrodepositing decorative and corrosion resistant chromium on a metal substrate which comprises electrolyzing an aqueous acidic solution at a temperature of from 100 to 130 F., less than 87.5 amp hours per square foot measured at the metal substrate cathode at a current density of from 100 to 350 amps per square foot, said aqueous acidic solution consisting essentially of chromium trioxide in a concentration range of 200-380 grams per liter, fluosilicate ions in a concentration range of 0.6 to 5.1 grams per liter, sulfate ion in a concentration such that the weight ratio of CrO /SO is between about to 1 and 230 to 1 and materials selected from the group consisting of sodium ion, ammonium ion and mixtures thereof, said materials being in the following ranges:

Grams per liter Na+ 2-24 NH 2-20 And mixtures thereof 2-24 2. The method as stated in claim 1, wherein the SiF concentration is between 0.6 and 3.8 grams per liter.

3. The method as stated in claim 1, wherein the SiF concentration is between 2.5 and 5.1 grams per liter, and the weight ratio of CrO /SO is between about to 1 and 230 to 1.

4. A method of electrodepositing decorative and corrosion resistant microcracked chromium on a metal substrate which comprises electrolyzing an aqueous acidic solution at a temperature of from 100 to 130 F., less than 87.5 amp hours per square foot measured at the metal substrate cathode at a current density of from 100 to 350 amps per square foot, said aqueous acidic solution consisting essentially of chromium trioxide in a concentration range of 200-380 grams per liter, fluosilicate ions in a concentration range of 0.6 to 5.1 grams per liter, sulfate ion in a concentration such that the Weight ratio of CrO /SO is between about 150 to 1 and 230 to 1, selenate ions in a concentration range of 0.001 to 0.015 grams per liter, and materials selected from the group consisting of sodium ion, magnesium ion, ammonium ion and mixtures thereof, said materials being in the following ranges:

Grams per liter Mg 2-24 Na+ 2-24 NH, 2-20 And mixtures thereof 2-24 8 References Cited UNITED STATES PATENTS 2,800,438 7/1957 Stareck et a1 204-51 XR 3,041,257 6/1962 Cope et a1. 204-5l 3,340,165 9/1967 Chessin 204-51 3,418,220 12/1968 Roggendorf 20451 XR FOREIGN PATENTS 220,060 5/ 1957 Australia. 1,150,854 6/1963 Germany.

OTHER REFERENCES Safranek, W. H. et al., Plating, vol. 47, pp. 1027-1031, September 1960.

GERALD L. KAPLAN, Primary Examiner 

